So, that's cosine theta d theta. And, well, if you do things with substitution, which is the way I do them, then you should worry about /2 the bounds for theta which will be zero to pi over two.
所以dx=cos，θdθ，如果用变量替换法来做，接下来就要考虑θ的取值范围,它是从0取到π
Many a time we don't know what we should do, but we still don't do it when we knew what we should d...
很多时候我们都知道要做什么，但是知道了该怎么之后，我们仍然不去做。
Many a time we don't know what we should do, but we still don't do it when we knew what we should d...
很多时候我们都知道要做什么，但是知道了该怎么之后，我们仍然不去做。
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